The explosive growth in the volume and variety of multi-media telecommunication applications continues to drive speed demands for internet traffic and motivate research in backbone fiber-optic communication links. Coherent communications and electronic digital signal processing (DSP)-based receivers have been accepted in recent years as the next-generation standards for long-haul systems due to their flexibility, scalability and ability to compensate for various transmission impairments, including fiber nonlinearity.
However, the proposed DSP algorithms for nonlinearity mitigation are not anticipated to provide orders-of-magnitude performance improvements. Consequently, new transmission media and multiplexing/demultiplexing techniques may need to replace standard single-mode fibers to achieve transmission performance beyond what the DSP algorithms can provide.
Because fiber nonlinearity places a limit on achievable spectral efficiency, large effective-area (Aeff) single-mode (SM) fibers have been designed for reducing nonlinearity penalties. One proposal for further reducing the nonlinearity of the transmission fiber is to utilize fibers that guide more than one mode at the operating wavelength. In these few moded fiber (FMF) designs, the effective area of the fundamental LP01 mode is higher than in single-mode fibers, and an optical signal launched into the LP01 mode will suffer lower nonlinear impairments. However this technique will only yield good performance if the power launched in the LP01 mode remains in that mode without coupling to the LP11 mode or other higher order modes (HOMs). If this mode coupling occurs, the optical signal will be degraded by multipath interference (MPI).
However, while single mode transmission in a few moded fiber (FMF) offers an improvement over current optical transmission systems, the spectral efficiency of an optical fiber increases slowly with increasing effective area. Another solution is needed to increase system capacity. Recent experiments have demonstrated that it is possible to transmit signals in more than one spatial propagation mode of a FMF using multiple-input multiple-output (MIMO) techniques. Typical few moded fibers can introduce coupling between the modes propagating in the fiber, resulting in the optical signals that will be degraded by multipath interference (MPI).